text:
How is the 737 main landing gear protected from weather?
I really like the 737, it is perhaps my favorite airliner. One curiosity of this aircraft (afaik, it is the only commercial airliner) is that the main landing gear has no cover:

Here is another picture from the inside:

In this situation, how does the aircraft protect the gear mechanism from weather/freezing?

why would an aircraft need to protect the gear from weather/freezing? when the aircraft is parked isn't the gear exposed anyway? how would that be different?

Because the outside temperature when cruising is well below freezing. If ice can form on the wings - can't it form on the exposed area?

Wouldn't the open holes cause more drag?

Open holes causes more drag but complexity and weight penalty of gear doors outweigh the drag.

Aircraft icing is not caused by ice in clouds- it is formed by supercooled water droplets (Source). They freeze on impact with a surface that has some location with zero flow velocity, most often on the leading edge of the wing.

Ice accumulates on every exposed frontal surface of the aeroplane – not just on the wings, propeller, and windshield, but also on the antennas, vents, intakes, and cowlings. It builds in flight where no heat or boots can reach it. It can cause antennas to vibrate so severely that they break. In moderate to severe conditions, a light aircraft can become so iced up that continued flight is impossible. The aeroplane may stall at much higher speeds and lower angles of attack than normal. It can roll or pitch uncontrollably, and recovery may be impossible. Source

The gear as such as is not worse off than normal landing gear. The B737 has wheel caps visible in your first photo to cover towards the outside, and is not allowed to fly without them. If icing had been a problem, the same concern would extend onto landing gear doors being frozen shut. Landing gears bays are unpressurised and very cold normally anyways.

the wheel caps cover the hub, I guess he is worried about the rubber.

The 737 Configuration Deviation List (CDL) does allow the aircraft to be dispatched without the wheel covers installed. Depending on the aircraft model and exactly what's missing there are performance limit requirements and the antiskid on that wheel may have to be deactivated with those requisite limitations.

Most of the aircrafts have covers (caps) so as to isolate the landing gear from the air around so as to prevent it from icing. Commercial airliners reach altitudes to the proximity of 35000 feet (and maybe more in some cases) and temperatures could drop 80 degrees below zero. In such cases, if the landing gear has caps and is isolated from the free stream, the heat retained in the landing gear bay from the hydraulics and the friction in the tires while takeoff does keep the landing gear warm and hence protect it from icing (source).

Heat from the plane’s hydraulic lines in the wheel well, as well as
  heat retained in the tires, could have helped the stowaway survive as
  the aircraft climbed to altitudes with sub-zero temperatures, the FAA
  reported.

Now, for airplanes like the one shown above, there is a possibility of the landing gear getting iced. When this happens, it might cause a range of problems. Problems ranging from skidding while landing to jammed landing gear. Usually, before landing when an airplane descents, the rise in temperature is good enough to deice the landing gear, but if that does not happen then pilots need to tackle with the icing problem in flight itself. Source

When surface temperatures are low enough for frozen deposits to be
  present, consider whether the mechanisms might have become frozen
  during flight as a result of prior extended taxiing through slush or
  wet snow. Flight above the freezing level for prolonged periods may
  result in the landing gear becoming frozen in the retracted position.
  This is less of a problem on most modern commercial transport
  aircraft, where the gear bays are largely enclosed by doors unless
  retraction/extension is in progress, but is a relatively common
  occurrence on aircraft such as the LOCKHEED AC-130 Spectre. If it is
  suspected that taxiing through slush or wet snow may have led to
  deposits adhering to the landing gear assemblies, then it may be
  advisable to cycle the gear after initial post take off retraction to
  try and shed any deposits.

There is no way the heat of the tires or hydraulic lines in a wheel well are going to help someone survive in a wheel well. The main reason landing gear has doors is to streamline the airplane and reduce drag. That's it. If it's -80 outside the wheel well, it will very shortly be -80 inside the wheel well. This poor guy was laying against the pneumatic supply ducting, the only really warm spot in a wheel well and it didn't help him one bit. http://www.nydailynews.com/news/world/dead-body-found-landing-gear-new-york-tokyo-delta-flight-59-article-1.171196

I gave the source which claimed that..!!
Secondly, the reason caps are there, is because to streamline flow, and to prevent icing of the landing gear (Again please have a look at the source). I did not mention the streamlining of the flow because it was not required to answer the question.
p.s. It feels bad when people dont read the complete answers and pass judgements :(

Your source is from the LA Times quoting someone from the FAA. With 30 years in this industry I can assure you that neither of these sources is reliable. You may however continue to believe what you wish. I hope however that you choose to never try this on your own or you will end up like the stowaway in the DL wheel well. I also don't know where this protection from ice thing comes from but carry on with that delusion as well.

Let's say SFO-NRT is 10+ hrs. How insulate is landing gear door?

Wheel well temperatures even on aircraft with doors get well below freezing in flight. On the 737, the only exposed part of the gear is the tire sidewall and aerodynamic hubcap.


text:
What are the differences between hand and hold luggage screening?
I have googled a little about the type of security screening done to hold and hand luggage.  From what I have seen the hold luggage is clearly subject to more rigorous screening, including tomography should it raise suspicion.
However, in light of the recent announcement for passengers to travel with charged mobile devices and be able to switch them on, I have become slightly confused as to why an explosive device would not cause an issue in the hold (I presume passengers won't be asked to remove all electrical items from their hold luggage and switch it on!).
Is the feeling that a small explosive device could do more damage in the main cabin, whereas in the hold the aeroplane's structure would absorb it?  Or is there really a "foolproof" way for the hold luggage scanners to detect whatever is being searched for ?

even a small explosion will tear apart a pressurized airplane if it is against the wall, when you hold it you can put it there, when it's in the hold you are taking a gamble

Bombs in the hold can certainly bring down a plane: Pan Am Flight 103 comes immediately to mind.

I personally do not see why this question has been flagged as off-topic. To me is clearly about aviation security.

"in light of the recent announcement for passengers to travel with charged mobile devices and be able to switch them on": I don't know what you are talking about at all. Presumably in the US, but the site is international.

As ratchet freak pointed out you do have some more flexibility in ensuring that a small cellphone-sized explosive device is positioned to cause maximum damage if it's in your carry-on baggage as opposed to if it's in checked baggage/cargo. As such there is an inherently higher risk here.
Counterbalancing that risk is the fact that we already have good screening tools to detect explosives or other anomalous things (like say knives or tear gas) concealed in what purports to be an electronic device:

Electronic devices are subject to X-Ray inspection.
Trained operators can spot quite a few anomalies using these "classic" tools (though rechargeable batteries can often look like explosives).
Explosive Trace Detection machines are already a part of security screening.
There are also swab machines which operate on a similar principle. They are not foolproof (they have false positive results occasionally, and may miss well-packed explosives), but they're a good tool.
Airport security has already asks to see suspect devices power on.
I had an old Compaq laptop with a completely useless battery, and security would let me plug it in if the battery was dead.

These same techniques (and others) are available for use on checked baggage.
Checked baggage may also be opened for inspection, which could conceivably include powering on any electronic devices in checked baggage (but note that the TSA recommends you not pack your electronics in checked baggage).

It's worth pointing out that, at least from a technical standpoint, faking a "power-on" is trivial with today's technology. Unless they're making folks do a full functional demonstration of the device in question a motivated individual with malicious intent would have no problem making a device that's mostly filled with explosives but can still "power on" and display a home screen, which makes the additional security benefit here dubious, at least in my estimation.

The rules as presented by many airports simply say that electronics should go in carry-on luggage. Not worded as mere recommendation.

@JanHudec As far as the TSA is concerned it is a "mere" recommendation (and you'll note the TSA info is from their Claims Management branch - the "Don't blame us for stealing your stuff" branch). There is no (TSA or DHS) prohibition on electronics in checked baggage that I'm aware of, however there is a DOT prohibition on spare lithium batteries. Individual airlines and airports are of course entitled to set their own policies, which may differ from the minimum requirements.

I live in Europe where instead of one TSA we have separate police with it's slightly different rules in each country. The airports and airlines then obviously prefer to stay on the safe side instead of dealing with the details.

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text:
Why was this commercial jet accompanied by small plane near Seattle?
From the South Beach Campground on the western shore of the Olympic Peninsula in Washington State we observed this unusual formation:

I only had a Sony RX100 at hand but perhaps the image quality is sufficient to recognize the airplanes.
The picture was taken on July 23rd, at 20.20h PDT (the image timestamp is 6 minutes off), looking roughly NNW. I don't dare guess the altitude, but to the bare eye the small airplane was a mere dot. The two planes flew in formation in a circular counter-clockwise pattern around the north-eastern part of the Olympic Peninsula. They re-appeared two or three (I think three) times. They appeared to fly at low speed.
We realized that we were only approximately 150 km west of Seattle, the Boeing headquarters. Was this a test flight? Which planes are involved? What is the function of the small accompanying jet?

Possibly two aircraft at different heights in a holding pattern?

@MikeBrass That didn't occur to me -- may be; but the relative position was almost identical each of the three times we saw them which appears unlikely for two unrelated aircraft, doesn't it?

The smaller aircraft was probably filming the larger one for promotion/advertisements.

It's a tow plane :D

Did the larger plane ever eclipse the smaller from your point of view? If so, you probably appear in their photos!

@TomMcW Is that like an AirTugBoat?

It's a Fedex 777F N894FD in (probably) pre-delivery aerial photography/test flight accompanied by one of Boeing's chase aircraft.
After some maneuvers (shown below), the plane headed to MEM, Fedex's "SuperHub".

(Source)
The flight on flightradar24: https://www.flightradar24.com/2019-07-24/03:16/12x/FDX9032/2168b4b1 and flightaware: https://flightaware.com/live/flight/FDX9032/history/20190724/0215Z/KPAE/KMEM

I'd say that's clearly a business jet not a T -33

Indeed, the engine position and T tail distinguish it.  But worth remembering the Learjet's inspiration by 1950's military jets - viewed overall, the P16 often considered the most direct inspiration actually looks more like a T33 than it does like a Learjet.

I give you the accepted answer because of the flight radar link, even though the leading plane does not look like a T-33 to me either. Very nice to see the flight pattern, thank you for the research. They must have got a few nice pictures circling Mount Olympus in the evening sun. How did you identify the plane? Did you just go to FlightRadar24 and observe all airplanes in that area around 20.20h PDT, spotting the loops I described?

@PeterA.Schneider I had a lot to work with. I knew the time, date and location, the aircraft type (777) and that it most probably have taken off from Everett since it was a delivery or photography flight.

In Soviet America, chased plane chases you!

Watching the flightradar24 link the chase plane never shows up as its own aircraft. From what I understand "formation flights" frequently only have one operating transponder, but since the aircraft was delivered to MEM shouldn't the chase at some point show up as a distinct track?

It should be mentioned that those circles are near the campground mentioned by the OP

It looks like an air-air photography trip by Boeing - the lead airplane is a LearJet, a type often used for this type of job with a turreted camera sticking out of the floor for views to the rear, ie for head-on shots of the target aircraft which in this case is a 777.

That's a 20 series Lear. Total shot in the dark, but my guess is that the exact airplane is N49WA.

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text:
Is it legal to land when an airfield is reporting low IFR due to fog with a visual approach?
Suppose you are on an IFR flight plan and are coming in to land at your destination.
The weather at the destination is reporting 600RVR due to dense fog.  The fog isn't very high as you can see through the fog while on approach and can see the approach lights and part of the runway lights.  The weather above the fog layer is VMC. The lowest visibility for an ILS approach that can be executed by the pilots is 1800RVR.

Can the pilots accept a visual approach even though the airport is reporting IMC?
Are there other factors that should be considered on an approach like this?

The question in the title and the question in the body are quite different.  In the title question you imply legality of the pilot executing a actual landing with the field in sight despite weather being reported below minimums.  In the second part, you question ATC being able to offer the visual approach.  To me it is unclear what you are actually driving at.

I am looking to answer if I can get and accept a VFR approach clearance under that scenario.

If you have the field in sight and are in a position to land safely I believe that trumps and called weather.   But why would the "visual approach" distinction matter?  You have been flying a published instrument procedure prior, correct?

Not necessarily.  The pilot is in VMC the while time on an IFR flight.  No instrument approach was flown.  Radar vectors towards the airport and the pilots told ATC the field was in sight.

If you are on an IFR flight plan, Approach is going to ask you if you have the field in sight. If not, they are going to hand you off to Tower. I’ve explained Tower’s response in my answer. If you cancel your IFR flight plan, you can do anything you want. If it is an emergency and you have to land, then you have to land. If you do not have an emergency and land safely anyway, it’s probably a case of no harm, no foul. If there is an incident, the FAA can use Part 91.13 against you.

@wbeard52 Note that a "VFR approach" and "visual approach" are two very different things!

OK, I get where you are coming from now.  ATC is bound by rules, and cannot suggest to pilots any ways to get around them.  Your best bet to let them off the hook if you know you can land safely is to cancel IFR.  (see my comment on J Hougaard's answer below...)

@MichaelHall Actually, if you cancel IFR, ATC would not be allowed to clear you (can't approve VFR when weather is below VMC). But if you are IFR and report field in sight, you can fly a visual approach (which is an IFR procedure!)

@J. Hougaard, In the United States the net effect of either option is the same.  The pilot is telling the controller "I can see the runway and land safely, I no longer require or desire radar services."  VFR is the default when an IFR clearance is cancelled, ATC does not "clear" a pilot for VFR operations.

A visual approach can not be authorized by ATC during true IMC.

5–4–23 Visual Approach
  a. A visual approach is conducted on an IFR flight plan and authorizes a pilot to proceed visually and clear of clouds to the airport. The pilot must have either the airport or the preceding identified aircraft in sight. This approach must be authorized and controlled by the appropriate air traffic control facility. Reported weather at the airport must have a ceiling at or above 1,000 feet and visibility 3 miles or greater. ATC may authorize this type approach when it will be operationally beneficial. Visual approaches are an IFR procedure conducted under IFR in visual meteorological conditions. Cloud clearance requirements of 14 CFR Section 91.155 are not applicable, unless required by operation specifications.

I was in this situation at the end of my very first solo cross-country flight many years ago.
I was in a Piper Archer, in the middle of the night, after flying from Marathon, FL to Lexington, KY (with two stops in between).
Forty miles out, the field was beautiful VFR with calm winds.
Thirty miles out, ATC advised that the visibility had dropped to 6SM.
Ten miles out, ATC advised that the visibility had dropped to 1SM, so I requested a contact approach to expedite my arrival before the visibility got even worse.  That's when they advised that it had actually dropped to 1/2SM, which is lower than the 1SM requirement for a contact approach, and lower than the 3/4SM minimum for the approach in use (ILS 22).
I requested vectors for the ILS 4 instead (minimum visibility of 1800 RVR), and after a little bit of mental gymnastics by the controller, they said "Sure, why not?  Nobody else is out here anyway, so we can switch the airport around."
By the time that I was on final, they advised that RVR was down to 800 feet.  I could see the runway lights perfectly, but could also see the fog sitting over the airport.
I momentarily debated what to do (I was tired, and didn't really want to divert...  at least I had made the extra fuel stop and had plenty of fuel left though!) before realizing that while I could legally continue the approach, I would almost certainly have to go missed at a very low altitude when I entered the fog since 14 CFR 91.175 states (emphasis mine):

§91.175   Takeoff and landing under IFR
  ...
  (d) Landing. No pilot operating an aircraft, except a military
  aircraft of the United States, may land that aircraft when—
  ...
  (2) For all other operations under this part and parts 121, 125, 129,
  and 135, the flight visibility is less than the visibility prescribed
  in the standard instrument approach procedure being used.

While the flight visibility was fine at the moment, as soon as I entered the fog, it would drop below minimums, and I would not be allowed to land.
Note also that this was for an actual instrument approach.  If it was visual, than I would have also had to avoid the "ground cloud" (aka fog) by the cloud clearance requirements specified in 14 CFR 91.155 by staying 1000 ft. above it.
If you have reason to believe that this isn't the case (maybe it's patchy fog and it's clear at the beginning of the runway), then you can go ahead and fly the approach, and land as long as you don't enter the fog and lose the required visibility or airport/runway environment spelled out earlier in the same regulation.
For those that want to know the rest of the story, I ended up diverting and then had to divert from my alternate (which was clear when I started there but completely covered by unforecast fog when I arrived), and then fly a low visibility approach at a third airport.  Once I landed, I went into the FBO and slept for the rest of the night in one of their recliners.
I wouldn't recommend that most people try this on their first solo IFR flight though!!

Let us continue this discussion in chat.

Yes, a visual approach can be conducted, if the pilot determines that the conditions are good enough.

6.5.3.3 An IFR flight may be cleared to execute a visual approach provided the pilot can maintain visual reference to the terrain and:
a) the reported ceiling is at or above the level of the beginning of the initial approach segment for the aircraft so cleared; or
b) the pilot reports at the level of the beginning of the initial approach segment or at any time during the instrument approach procedure that the meteorological conditions are such that with reasonable assurance a visual approach and landing can be completed.

ICAO DOC 4444
It is fairly common where I work for shallow patches of fog to develop during early morning and in the evening just after sunset, especially on days with otherwise clear weather. This can result in fairly low RVR values for some parts of the runway, even if the overall visibility is good and there are no clouds. If an approaching pilot has the runway in sight and decides he wants to fly a visual approach, there is nothing wrong with that.

I used to be faced with that a lot when bush flying on floats. In the fall on a clear sunny morning after a cold front passage, the warm lakes would all be fogged in with the fog tops level with the tree tops.  Sometimes it would be clear enough at one end of a long lake to land, then taxi into the fog following the shore line to the amazed customer's camp.

NAS Whidbey Island was sea level and right on the water. In late summer particularly the fog would roll in and obscure the tower and half the field, while it was clear and a million everywhere else. Reported weather was zero zero, and being single piloted we weren't even allowed to commence an instrument approach if weather was below minimums. However, I would call approach at 20 miles and ask for a visual. They would tell me to "report VMC, field in sight" which I did immediately, and as soon as I did I would be cleared.

FYI, the original question is tagged [tag:faa-regulations], and you provided an ICAO reference, which may not be what they are looking for.

As Dean F. points out, a visual approach clearance may only be issued if the airport is reporting VFR (or if we don't have the airport weather available to us so we take your word for it that it is VFR). That's in the 7110.65 7–4–3b.
I'm not familiar with all the pilot regulations, and especially with any OpSpec that might prohibit this approach, but I will ask whether having the runway environment in sight negates any otherwise restricting rule. 14 CFR 91.175 quoted by Lnafziger references flight visibility, not ground visibility or RVR; surely the pilot is the reporting source for flight visibility, and seeing the runway lights at sufficient distance overrides any other consideration? Perhaps not; you're the PIC and I won't force you to land if you don't want to.

If you decide that you can't perform a visual approach or an instrument approach, and you can't do a contact approach because reported ground visibility is less than one mile, and you can't cancel IFR and proceed under a Special VFR clearance because, again, ground visibility is less than one mile, the only option left (if you need to land at this specific airport) is to declare an emergency. The .65 discusses SVFR operations and includes this extensive note under paragraph 7–5–7 "Ground Visibility Below 1 Mile" (emphasis mine):

NOTE- Clear an aircraft to land at an airport with an operating control tower, traffic permitting, if the pilot reports the airport in sight. The pilot is responsible to continue to the airport or exit the surface area. 14 CFR Section 91.157 prohibits VFR aircraft (other than helicopters) from landing at any airport within a surface area when ground visibility is less than 1 mile. A pilot could inadvertently encounter conditions that are below SVFR minimums after entering a surface area due to rapidly changing weather. The pilot is best suited to determine the action to be taken since pilots operating under SVFR between sunrise and sunset are not required to be instrument rated, and the possibility exists that flight visibility may not be the same as ground visibility. 14 CFR Section 91.3 authorizes a pilot encountering an inflight emergency requiring immediate action to deviate from any rule of 14 CFR Part 91 to the extent required to meet that emergency. Flight into adverse weather conditions may require the pilot to execute the emergency authority granted in 14 CFR Section 91.3 and continue inbound to land.

The context here is an aircraft already operating SVFR within the surface area when the visibility drops to below one mile, but the suggested solution (declare an emergency and land anyway) seems relevant.

Perhaps the best solution would be to ask ATC if there's a helpful wing-mounted-turbojet aircraft somewhere on the field who could be enticed into doing a high-speed taxi on the runway in the hopes of blowing the fog away from the RVR sensor. (NB: I am not actually recommending this, though I have heard rumors of a similar situation when an air carrier was trying to depart and the RVR was too low.)

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text:
What are the major hazards in operating a PT6 Turbine Engine?
I am just in the process of doing a bit of study in the hope of doing my Basic Gas Turbine Endorsement in the next few months, and just wanted some feedback/comments/discussion on the major hazards associated with starting, managing and shutting down a PT6 Turbine Engine (PT6 as it is the most common used Turbine Engine on an aircraft I will touch for this endorsement).
Mainly, what I am looking at is during the start - If the EGT raises above the evil 700* mark, what is the best course of action to prevent "melting the turbine"?
During Climb/Cruise Operation - What are the most common failures, and their indications?
Maintenance/Shutdown - What are the most common methods of recording Cycles? By landing or by Start/Shutdowns?

This is kind of a broad question with a lot of sub-questions. Is it possible to focus it a little more?

Or split to three separate questions? Also the first part is probably best answered by looking up appropriate procedure in your pilot operating handbook, flight crew operating manual or quick reference handbook you should have for the aircraft you are going to use (and we can't look it up for you because you mentioned the engine, but not the aircraft).

You MUST NOT allow the EGT or ITT (whichever is applicable) temperature limit to be exceeded in any turbine engine. If it gets close to occurring during start on a PT-6, you need to abort the start by stopping fuel flow into the engine, then conduct the appropriate checklist for an aborted start. Unlike the Honeywell / Garrett TPE single-shaft engines, the PT-6 normally won't get close to the temperature limit on start. The Garretts I used to fly in Metroliners had to get right next to redline and stay there on every start. One reason that PT-6 engines are so much more relaxing to fly.
On takeoff / climb, just don't exceed the maximum temp limit; regulate the power lever(s) to keep below it. I've never had a failure of a Pratt except for bleed air leaks.
Airlines count cycles as a start, flight, and shutdown. If one of these did not occur, you did not have a cycle.

Do not mix EGT and ITT. EGT is exhaust gas temperature and ITT is Inter stage temperature.
Normally when starting a PT6 you should watch ITT and depending which PT6 model you use maximum ITT may vary but it`s around 1600 degrees Fahrenheit.
I work for Pratt & Whitney and PT6 is probably the best engine ever made in it`s class as someone mentioned. When starting a PT6 the engine will not get close to the maximum allowable ITT unless you have a problem.
Good engine choice hope you will have good time flying with this engine.

Hello JoBlow and welcome to Aviation.SE. Would you mind to add some interpunction into your answer to make it better readable?

The evil mark you are talking about varies. There ara all kinds of PT6, ranging from around 500SHP to 1700SHP, each of them with different limits, including ITT limits. Which PT6 are you talking about? Overtemperature during start means that something is wrong with the engine. Either the starter did not crank the engine fast enough, the engine hot section is worn out or something is wrong with bleed valves/FCU.

"Starter did not crank the engine fast enough" might be an issue with the air source for the starter, not the engine.

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text:
Why do some airfields not have manned ATC towers?
I was wondering why some airfields, for example Tatenhill Airfield (EGBM), don't have anybody in their ATC towers?

You might get more understanding by editing your question to explain why you think there would be someone in the tower?

as far as I can see, Tatenhill Airfield doesn't even have a building that resembles a tower. https://goo.gl/maps/mMHja

In the case of Tatenhill, the field was previously used by the RAF. Evidently the original control tower was replaced by a previous operator, which explains why there is still a control tower there. But currently it is a private field, where the amount of traffic doesn't warrant operating the control tower.
Information from NATS lists a Tatenhill Radio frequency that provides A/G (air to ground) service during certain hours. YouTube videos suggest no tower services.
There are many airports in similar situations, that do not operate a control tower (whether or not they have one), or only operate at certain times, when there is not enough traffic for a control tower to be needed. VFR rules give pilots the responsibility to "see and avoid" each other in these areas, and announce their intentions on the appropriate frequencies.
Also, see the related questions to the right, especially: What's the process for an uncontrolled airfield to become controlled/towered?

It shows in Blue  on the VFR maps, which is supposed to indicate a manned airport.   Magenta icons are supposed to indicate untowered airports.  Is the map wrong?

I understand, thats a great help, thankyou
and thankyou abelenky

@abelenky It appears that there is a frequency but not providing tower service, I've edited with more info.

One of the biggest reasons to not man a tower is (not unsurprisingly) costs.
Many remote airports only have a few movements per day, and a permanent staff in the tower makes it even more difficult keep the operation economically viable.
For this reason, remotely staffed towers are being developed now, and for instance Örnsköldsvik airport (ESNO) in Sweden has just received approval from the Swedish Transport Agency to operate with a remote tower:

The system’s sensors are installed at Ornskoldsvik Airport but traffic
  is remotely controlled from Sundsvall, Sweden.


text:
Are multi-crew operations in a single-pilot aircraft counted towards ATPL requirements?
I thought that you had to perform all your ATPL multi-crew time in aircraft with a certificated minimum of 2 crew, but I've heard of some pilots using time in single-pilot aircraft towards their EASA ATPL licence. Is this true? If so, could someone explain how is this possible?

According to the UK CAA you need:

500 hours in multi-pilot operations on aeroplanes;

Notice that it does not say anything about the aircraft being multi-pilot/certified for two crew members.
An air carrier can specify in their ops manuals that, for their operations, they require a particular airplane to be operated with two crew members even if it is only type certificated for one crew member.  If their manual is approved, then it is now a "multi-pilot operation" regardless of the type-certification status of the airplane.


text:
How are airlines able to work out gate allocations so far in advance?
When flying on a long-haul flight, you can often see gate allocations for when you land a hour or two before actually arriving at the airport, including transfer information and connecting flights. However, with so many aircraft and frequently changing situations, how do they manage to allocate these gates so far out in advance?

I'd point out that the gates do change with delays/early arrivals sometimes. But generally they are pretty good. I'm curious as well

some airliners just lease/reserve a set of gates for their own planes

There's also an element of 'once you've allocated a gate, stick to it' with this. Meaning: there's a fixed set of costs in changing gates, even if the benefits of a sudden gate change might look greater on paper. So, airlines don't change gates real time, all the time to achieve full optimization. And there's always (or almost always) a bit of excess capacity to deal with changes. At airports where there is no excess gate capacity at all, things can get pretty nasty quickly.

Route times are pretty well known. If you know when the plane should depart and how long the flight should last, it's pretty easy to calculate when it will be arriving. Give a few minutes leeway on either side, and you should be good. If the plane is not there on time, you send it somewhere else.
Most large aircraft have some sort of ACARS that allows them to talk to company dispatch and let them know if they're going to be early or late, among other things. Barring that, they almost always have a company radio frequency that they can relay messages on one way or another.

It also helps when an airline owns (or is the sole lessee for) a terminal at a given airport. Jet Blue knows what's going on at their terminal at JFK, and if changes to gate assignments need to be made the dispatchers can inform your flight crew, who can in turn pass that information along to you.


text:
How do aircraft keep fuel from freezing while in flight?
On aircraft which fly above 30,000 feet, the temperature can often can get to temperatures below Jet-A's freezing point.
For example: at 36,000 feet the standard outside air temperature is -56.5°C (-69.7°F) and is well below Jet-A's freezing point of -40°C/F.
What measures are put in place on aircraft to mitigate the risk of the fuel in the tanks freezing and preventing the engines from operating?

Much like water, exposing fuel to below freezing temperatures does not instantly make it freeze.  It takes some amount of time, and as long as the fuel is not exposed to below freezing temperatures long enough to make it freeze, it won't be a problem.
How much time?  That depends on a number of factors like:

Type of fuel

Jet A (common in the US) has a freezing point of -40 C
Jet A1 (common outside the US) has a freezing point of -47 C
Jet B (common for some military aircraft and very cold airports) has a freezing point of -60 C

Initial fuel temperature

If you fuel in the middle east during the middle of summer, the fuel will be a lot warmer than when you fuel in Siberia in the middle of winter.  Warmer fuel will take longer to freeze.

Outside air temperature

Much like trying to freeze water to form an ice cube, the colder the air around it, the faster that it will freeze.

Aircraft speed

The faster that the airplane goes, the more friction there is as the air flows over the wing.  This heats up the air close to the wing and makes it take longer for the fuel to freeze (see previous point).  The temperature at the wing is known as TAT:
For example, at an actual temperature of -56.5 C (SAT) and Mach 0.72 the TAT is -34 C, but at Mach 0.80 the TAT is -29 C.
For more information about SAT -vs- TAT, see my answer about Air Data Computers.

Quantity of fuel

Much like a small pond will freeze before a large lake, in order to get fuel to freeze you need to lower the temperature of all of it.
The more that there is, the longer it will take.

Aircraft design

Some aircraft run hydraulic lines close to the fuel so that the heat will help slow down the freezing.
Some aircraft constantly pump fuel to keep it moving and mixing if it tends to freeze in one part of the tank before the rest
Some aircraft use fuel to cool the engine oil using a heat exchanger, and return the warmed fuel to the tanks
Many other design factors

In general, fuel will not freeze in the typical amount of time that an aircraft is aloft.  If particular routes pose a problem, there are several options that the operator and pilot have:

Use a fuel with a lower freezing point
Fly at a lower altitude where it is not as cold
Choose a route with warmer temperatures
Increase the speed of the airplane
Carry extra fuel
Transfer fuel in a way to keep fuel warmer.  Typically this is done by transferring fuel from the relatively warm fuselage tanks to the wing tanks which are exposed to more of the cold air.

sometimes you get water in the tank and if that freezes you have ice crystals at the bottom of the tank where it may cause havok

@ratchetfreak That's what fuel heaters and/or prist is for!

@larryw Prist does not keep fuel from freezing.  It keeps the water that is suspended in the fuel from freezing.

In addition to the excellent points, I'd mention outside air pressure. Yes, it's cold at high altitudes, but not like it being cold at sea level; it's cold because there is an insufficient amount of air to conduct heat into the object being measured (or doing the measuring). By the same token, that air isn't conducting heat away from the aircraft as fast as cold air at sea level would be. So while the wings are in -40*C air, they aren't equalizing to that temperature by conduction as fast as they would be if parked at McMurdo Station Antarctica.

Would you even be able to fuel a plane in Siberia in the middle of the winter without preheating the fuel?

@sean That's one of the areas that they would use Jet B.

Most Jet engine fuel systems have means to heat the fuel to prevent it from freezing. Typical heat exchange methods followed are by employing hot air or hot oil taken from the engine.
Reference: FAA Aviation Maintenance Technician Handbook, Chapter 14: Aircraft Fuel System

or putting the AC's heatsink under the tank

If I recall correctly, the couple of A380s that suffered engine failures back in 2009 or 2010 did so because the fuel/oil heat exchange manifold iced over.

@ratchetfreak: Not generally a good idea, given that most of the flights flown by any given type of jet (unless it's a type specifically specialised for cold-weather operations) are not ones where the fuel would be at risk of freezing.

Since the airplane is in motion, so is the fuel. That principle alone keeps it from freezing. As the fuel agitates, the molecules are prevented from freezing because of motion. This only works to a certain degree.

Can you add some sources for that?  I would think that it might not be enough movement to affect it very much.


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What should you do if you are forced to enter restricted or prohibited airspace?
Apparently, even if you have declared an urgent situation or an emergency, you cannot enter restricted or prohibited airspace, and ATC may not be able to clear you into the area. So, my question:
If you had a serious condition onboard an aircraft (say severe icing) and high terrain (I know, you probably haven't planned) in all directions except for restricted or prohibited airspace, what should you do to try and make sure that people are aware of your situation and can try and minimise the risk of doing so?

Disclaimer: not a real pilot. I'd do squawk 7700, explain situation to ATC and would collect as many evidences as possible (photos, etc) to prove you were forced to enter restricted/prohibited airspace.

This will probably only be a problem when you are in a small private plane, where indeed terrain plays a role. Then the only case is explaining your problem as clear as possible, and indeed take photos.

I agree with the no pictures part.  In an emergency, you have your hands full already.  Worry about legal issues when you are safe on the ground!

As PIC you can deviate from any FAA regulation as necessary to maintain the safety of flight. Like the comments above and answers below, first deal with staying alive. You might get intercepted and you might be greeted w/guns on the ground but you will be alive. You will then additional years left on your life to deal with the ensuing paperwork and possible legal battles. I will say I did get priority service and a greeting party for an instrument failure in IMC, it amounted to a phone call that ended with "You did the right thing to declare." (Wasn't restricted airspace though just Class C)

@p1l0t Keep in mind that very often special use airspace (particularly restricted and prohibited area) are governed by agencies other than the FAA. And those agencies don't always agree with things like the PIC's emergency authority.

Yes that is true, just because you don't give up your pilot's certificate doesn't mean that you can't be arrested, fined, or imprisoned by the controlling agency or an extension of their authority.

@Lnafziger of course, I was thinking of taking photos if the situation allows to do so. Unlikely, I know. I read about a RTO experience a while ago and I remembered that the pilot took some photos to the circuit breaker panel to document the fault that caused the RTO.

@orique Yeah, that's a great idea, but he was on the ground after the emergency was over then!  :-)

Even if ATC can't legally give you permission to enter prohibited airspace (they sometimes can for restricted airspace by the way), if you declare an emergency they will still coordinate with the controlling agency which will help to keep you safe.
Squawk 7700, fly as high as possible, turn on all your lights, broadcast your intentions on 121.5 if you aren't talking to ATC, and don't make any sudden dives toward the ground/buildings, etc.  This will all help to show that you aren't trying to "sneak in under the radar" and can only improve your situation.
Don't be surprised if you get intercepted though, and make sure that you talk to them on 121.5 if you do.  Know your intercept procedures (section 5-6-2) and follow them exactly.  They will most likely try to help you as much as possible.  You will still face a lot of questions and probably even certificate action when you get on the ground though because, as you said, poor planning is no excuse....
Also, this applies in the US. Some other countries (possibly Cuba, Iran, etc.) tend to be far more sensitive to issues like this and may "shoot first and ask questions later".

Some other countries (possibly Cuba, Iran, etc.) may "shoot first and ask questions later". : As far as I know, it is the United States navy that shot down an Iranian civilian passenger flight on 3 July 1988 in the very same way that you wrote about Iran. I am deeply sorry about your unilateral judgement.

@Mostafa That was a terrible incident, however my answer was specifically addressing restricted and prohibited airspace within the US. I would imagine that most countries in the Middle East would be far less tolerant of the situation that he asks about than here in the continental US which is what I described in my answer. Do you disagree?

@Mostafaııııııııııııııııııııııı: That was indeed a terrible tragedy. It is interesting to note that the onboard data recorders disagreed with the eyewitness testimony. The policy was to do no such thing. Now, what is the policy in Iran or Iraq?

Aviate, Navigate, Communicate.  In that order.
First, solve the in-flight emergency.  If that truly means penetrating prohibited or restricted airspace, do it.
Second, navigate to exit the prohibited or restricted airspace as soon as possible.
Third, Communicate to ATC what you are doing and why.  Follow instructions given if they don't conflict with your safety.
Finally, when on the ground, file a NASA ASRS report.  Explain what/where/why.  Pictures etc to document this are a good idea but not in flight.  You have an emergency situation... what are you fiddling with a camera for?  Take them on the ground.  If you have time to take in-flight photos (icing) they can be used against you unless you have a plane certified for flight in icing conditions.


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What is an Airworthiness Directive?
Aircraft which have been in service for a long time often have a lot of entries in their logbooks for Airworthiness Directives (ADs). What are they, and why are they important?

Straight from the FAA:

Airworthiness Directives (ADs) are legally enforceable rules issued by the FAA in accordance with 14 CFR part 39 to correct an unsafe condition in a product. 14 CFR part 39 defines a product as an aircraft, aircraft engine, propeller, or appliance.

When an aircraft or other component is found to have flaws or other issues, the FAA will issue an AD specifying a solution to be applied to the item. The AD must be complied with, often by a date or total time on the airframe. The action required could simply be an inspection (often recurring after, say, 100 hours of time in service), while in other cases requires modification, strengthening, or replacement of parts.
One of the more prominent cases requiring ADs to be issued was the V-tail Bonanza, which had issues with the tail's structural integrity under high loads.
The cost of complying with an AD falls on the owner/operator of the aircraft. Failing to comply with an AD renders the aircraft unairworthy. It also could incur FAA penalties, but more importantly, it could result in structural damage or failure, and in some cases ultimately cause a fatal accident.

Failing to comply with an AD has one other critical impact: It renders the aircraft unairworthy. Aside from the regulatory implications, most insurance companies void coverage if the aircraft was operated while not in an airworthy condition. So if you failed to comply with say AD 71-21-08 (that requires changing a fuel selector cover), and your landing gear collapsed on your next flight (clearly not related to the cover) your insurance company may just say "Tough rocks pal!"

Hah, yeah, totally overlooked that. Really good point; I've updated the answer.

An Airworthiness Directive is a legal notice, issued by either the manufacturer and/or the FAA, alerting an owner to an issue with the aircraft or system which presents a hazard to flight safety as well as instructions on how to remedy this problem.  It's similar to a recall notice on an automobile with the added mandate by the government that the aircraft is not airworthy until the AD is complied with.
It is very important that the owner retain records in the aircraft's maintenance logbooks that affective ADs have been complied with by an authorized AMT.  This is one reason that logbooks are so valuable to a prospective buyer and a big red flag and loss of value to a potential seller if they are missing, as any new owner would then have to spend their own time and money to ensure that the airframe complies with all affective ADs and would not be considered airworthy by the FAA until this was accomplished.


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How can a passenger tell if a flame-out / engine failure has occurred?
Reading this page, a retired American Airlines pilot quotes:

We tell passengers what they need to know. We don’t tell them things that are going to scare the pants off them. So you’ll never hear me say, ‘Ladies and gentlemen, we just had an engine failure,’ even if that’s true.

Given this, how can an observant passenger sitting over wing (seeing most likely very little of the engine), determine if a flameout or shutdown has occurred from observation?
This is applicable to an aircraft not unlike the Boeing 737 (which I fly on often).

As a passenger on a plane, the best thing you can do to enhance safety is to rest and be fit for your way from the airport - sorry but that's it. Feel as comfortable as you can in this seats rather than being stressed listening and looking for every detail.

Not all captains will keep this mum. Quote from the captain of British Airways Flight 9: "Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress." (source: http://en.wikipedia.org/wiki/British_Airways_Flight_9#cite_ref-Air_Disaster_3-3)

@FredLarson Well, that would be more of an extraordinary circumstance. I'm referring to the more common single-engine failure that is trained for and has a known plan of action to handle in most cases.

What are you going to do about it? Demand to be let out before you die in a fireball?  The pilots are well trained for an engine failure, they will handle it perfectly well.

Mainly by the soon to happen descent and landing, probably at a place you weren't expecting to go.  :)
If you are seated behind the wing, you might be able to see smoke or flames coming out of the engine, but other times it will just quit with no visible indicaion, depending on the cause of the failure.
Sometimes (usually just after takeoff when the airplane is slow) it is obvious and you can tell by the change in the sound of the airplane (one side suddenly gets quiet or there might be a loud bang depending on why it failed), a sudden yaw in one direction (the nose pulls to one side), and an abrupt pitching down of the nose (done by the pilot to maintain flying speed).
Other times (like during a descent with plenty of speed and the engines at idle, or when the center engine of a three engine airplane fails) it can be hard for even the pilot to tell without looking at the instrumentation, so you probably won't notice it.
Another major clue would be when the seatbelt sign unexpectedly coming on, without an announcement or indication of turbulence, and the flight attendants suddenly putting things away and getting ready for landing, especially when you aren't close to your destination. Being briefed for an emergency landing is also a dead giveaway (no pun intended).

Failures still occur during level flight at cruising altitude; would those be detected similarly with the changes in sound, yaw, and a downward pitch?

Yes, but it wouldn't be as pronounced so it might not be noticeable from the cabin.  They would have to start descending before very long though, so if you aren't anywhere close to your destination and the airplane start a descent it's a pretty good clue.

Ingestion of foreign material into the engine could cause flames and sparks to spew out of it before it shuts down completely.

@flyingfisch True, added!

@flyingfisch But, equally, ingestion of foreign material into the engine could cause flames and sparks to spew out of it before it continues to operate just as normal, right?

@DavidRicherby True, but loud bangs can also occur without the engine going out either. ;-)

seatbelt lights can come on when the passengers are a bit to rowdy for the crew's taste in an attempt to control them

@ratchetfreak Yes, or it could just be turbulence ahead.  As I said, it's only a clue.  :)

@ratchetfreak "This is your captain speaking.  Due to some of the passengers being rowdy after I asked you repeatedly to stop, I am now pulling this plane over for a landing right here as promised until you all settle down."


text:
Is there a benefit to having the motors for a multi-rotor copter on the top vs the bottom of the frame?
There is a new quadcopter being launched on Indiegogo that has the quadcopter motors positioned on the top of the propellers as opposed to being positioned on the bottom.
Is there any advantage to this or is it just for looks?

Welcome to Aviation SE!  I suppose if flying indoors you don't risk hitting the rotors on the ceiling.  Other than that, it's just for looks.

I would suspect something with compression versus tension load on the motor shafts, but I don't think you can get definitive answer from anybody except the actual designer.

It could be just for looks, but rotor placement has a noticeable effect on the behaviour of the quadcopter.
The higher the rotors are on the frame the higher the center of lift will be (the center of lift for quadcopters is the center of the plane created by the rotors.) A high center of lift means that the frame will be more stable. A low center of lift means the opposite. The reason for that is that the quadcopter can better naturally deal with lateral rotational forces on its frame. The center of lift being above the center of mass means that the frame is "hanging" like a pendulum from it. If you apply a lateral force to it, it will naturally swing back to place and the more you push the "harder" it becomes to move the frame.
A center of lift that is in the same spot as the center of mass of the frame means that there is no pendulum effect. This is like balancing a ball on top of another ball. All lateral forces, either internal like variations on each rotors' force or external like wind perturbations have maximum effect in moving the frame.
Contrary to what you might think, stability is not something you want in a quadcopter. The more unstable the frame, the more agile it becomes. Small variations in rotor force will create large rotational movements. The penalty you pay for that (small, unpredictable external forces also create large rotational movements) you make up with processing power. Processors are now so fast and sensors so sensitive that they easily can compute the external forces and create a response fast enough to keep the frame flying.

Actually moving the centre of lift up or down does absolutely nothing to stability of quadcopter. There is no mechanism to shift or bank the lift vector when the aircraft is banked, so the lift vector passes through centre of gravity making the craft neutrally stable in either case. I also thought it affects stability before I was corrected by pdel here.

I think that what you are saying is correct only if the lift vector does not change direction (pdel comments on that as well.) This is not true for a quadcopter.

Sorry for the double comment but, take a look at this as well: http://eprints.qut.edu.au/33767/1/33767.pdf under "4.4 Design for Optimal Sensitivity"

I will believe you there is an effect that can tilt or displace the total lift vector when banking, but I don't see it and I didn't find any conclusion in that article (and most quads don't have flapping rotors, so the discussion from that point may not even apply).

I am not sure that I follow you to be honest. A quadcopter can rotate its total lift vector by rotating one or two adjacent rotors faster than the others. The lift vector will rotate towards the rotors that are spinning faster in relation to the weight vector that is always pointing down. This is how quadcopters move laterally...

Aircraft is stable if it returns to previous attitude after external disturbance without control input. That is only possible if the attitude affects the lift vector. In airplane it does, but I don't see any way it would do so in quadcopter independent of rotor placement, so quadcopter should be neutrally stable in either case. What rotor placement affects is how big torque given change in power causes (and best should be rotors in place with CG; it's simple matter of moment arms).

There are many more ways to affect the vertical weight distribution (and thus the stability) than just selecting down vs. upward props, so this is probably not the main design factor.

@RRR: Differential rotor speed will not rotate the lift vector, but merely shift it towards the rotor with the higher speed. Now the craft rolls, and that will rotate the lift vector.

@PeterKämpf Yeah, you are right.

@abey is right, but his answer is a little short. Let me give some background.
Propellers work by accelerating air. If you think of a stream tube through which all air flows that will pass through the propeller disc, the diameter of this tube is bigger than the propeller diameter itself ahead of the propeller and smaller behind it. Since the air flows faster behind the propeller, the same mass flow needs less stream tube cross section. The propeller sucks air in, so the speed increase starts at some distance ahead, and raises pressure in the disc, so more speed increase follows behind the propeller when the pressure $p$ in the streamtube falls to the level of ambient air pressure $p_{\infty}$.

A strut placed ahead of the propeller will see less dynamic pressure than one placed behind it. Turn the picture above by 90° anticlockwise, and you have a quadcopter propeller arrangement. Make the initial speed $v_{\infty}$ zero and you have the situation for a quadcopter in hovering flight. Consequence: Propellers below the strut will have a higher efficiency.
Full disclosure: The picture above is idealized - in reality, the air behind the propeller will mix with air surrounding it, which will be dragged along. The streamtube idealization gives an easy way to calculate propeller thrust when combined with the assumption of identical air speed over the cross section of the stream tube, but in reality the speed distribution is uneven, with a gradual decrease with bigger distance from the axis of symmetry.

The main reason is related to how the arms impact the thrusters' aerodynamic efficiency. The impact is lower when the arms are on the air-intake side of the propeller rather than in the downwash. It's actually quite surprising that most multi-rotors do not have downward-pointing props. One reason might be that it becomes more difficult to mechanically implement the landing skids with downward pointing props.


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What are the accident rates for kit built aircraft vs. normal aircraft of the same model?
I know that some manufacturers both sell kit and finished version of their aircraft. For a fair comparison of accident rates, i was wondering:
What are the accident rates for kit built aircraft vs. normal aircraft of the same model?
Are the home-builds less safe than the company finished aircraft?

The Australian Transport Safety Bureau did a report titled:
Amateur-built aircraft Part 2: Analysis of accidents involving VH-registered non-factory-built aeroplanes 1988-2010
This report specifically covers this and found that the accident rate was three times higher in kit built -vs- similar factory build aircraft:

What did this report do
The ATSB investigated the safety history of amateur-built aircraft in
Australia through analysis of accident data held in the ATSB’s
occurrence database from 1988 to 2010. Comparisons were made between
accidents involving amateur-built aircraft and those involving similar
factory-built aircraft to help identify whether the rate and types of
accidents differed between these two groups of aircraft.
What the ATSB found
Amateur-built aircraft had an accident rate three times higher than
comparable factory-built certified aircraft conducting similar flight
operations between 1988 and 2010. The fatal and serious injury
accident rate was over five times higher in amateur-built aircraft, in
particular due to relatively more serious injury accidents. The pilots
of amateur-built aircraft involved in accidents were significantly
more experienced overall than factory-built aircraft accident pilots.
However, they were significantly less experienced on the aircraft type
that they were flying at the time of the accident.
Over half of the accidents were precipitated by mechanical events,
which were mainly complete or partial engine failures. Following the
amateur-built phase one test period, mechanical failures were still
significantly more common when compared with factory-built aircraft. A
quarter of accidents were from loss of aircraft control. Structural
failures were not common precursors in amateur-built aircraft.
Collision with terrain and forced landing accidents were more frequent
in amateur-built aircraft. Collisions with terrain, hard landings, and
runway excursions were more likely to result in a serious injury from
an amateur-built aircraft accident than for factory-built accidents.

On the other hand, the Experimental Aircraft Association says that homebuilt aircraft have an accident rate of less than 1% higher than the general aviation fleet in their FAQ, but this didn't compare homebuilt to factory built in similar models:

How Safe Are Amateur-Built/Homebuilt Aircraft?
Studies by FAA and the National Transportation Safety Board (NTSB)
show that Amateur-Built/Homebuilt aircraft have an accident rate less
than one percentage point higher than the general aviation fleet. In
fact, the accident rate for Amateur-Built/homebuilt aircraft is
dropping. The total number of registered homebuilt aircraft is
increasing by about 1,000 per year, while the total number of
accidents has stayed virtually the same. Another good barometer of
safety is insurance rates. Companies that insure both homebuilts and
production aircraft charge about the same rates for owners of either
type of airplane. That indicates a similar level of risk.

Should that be 'FAA' rather than 'EAA?'

@reirab No, EAA is the Experimental Aircraft Association and the specialize in aircraft like kit built airplanes.

ah, ok, thanks. I didn't notice the link there when I read it the first time. I guess that would have answered my question. I just noticed the quote immediately after it referring to studies by the FAA.


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What are the uses of gyroplanes?
From my understanding, a gyroplane (autogyro):

Is a rotocraft.
Uses an unpowered rotor for lift generation, instead of wings.
Uses a propeller for horizontal translation like an airplane.
Cannot take off vertically or hover contrary to an helicopter.
Can land vertically like an helicopter using rotor inertia.
Maybe simpler to build and maintain than an helicopter or an airplane.

(Source)
Are gyroplanes a technology that works but without particular application? Or do gyroplanes have well known uses for which they have significant advantages over airplanes and helicopters?

The POLICE marking on the side of the gyrocopter in the photo would suggest one accepted use, however limited. I think that apart from costs gyrocopters would probably have lots of good applications. A light fixed wing will often be cheaper with similar utility.

@JonathanWalters not if you want vtol capability.

@ratchetfreak Very true, and part of why I said similar.

Maybe good for low speed flight but lower cost than a helicopter? Don't they provide more lift at lower speed than a fixed wing? Just a guess. That would make sense why police might use them. Easier to patrol a small area than a fixed wing. I always thought they were kind of a hobbyist thing.

@JonathanWalters: Hum... that's what is advertised, however as they cannot hover, I don't see what they can be used for by the police in practical terms, maybe to watch traffic on roads, but they are slow too...

They make great props in James Bond movies

@mins I don't know the details of Police applications, though I know law enforcement that use Bell 206 and Cessna 182. I was primarily thinking of powerline or pipeline patrol. I know operations that use both the Cessna 177 and the Robinson R22; a gyrocopter like this seems to offer a compromise between the two, perhaps even in cost.

Maybe a compromise:

Compered to an helicopter is cheaper but less capable ("only" STOL capability, not VTOL capability)
Compared to a VLA more expensive and less efficient (speed,range,block fuel) to operate

Does anyone making comments here have any personal experience with gyrocopters? In order to make a non-speculative judgement based on experience, I mean.

Gyroplanes have quite a few applications, but the problem is that anything they can do, others (helicopters/fixed wing aircraft) can do better.

The main advantage of a rotorcraft is that they can hover at a location; the gyroplanes are unable to do this, which is kind of a deal breaker.
The main advantage of the gyroplane is that they cost less and are easier to maintain, compared to other type of aircraft. However, most of the autogyros available in market are small ones and as the size scales up, this may vanish.
The gyroplanes are used mainly for low speed surveillance operations due to their low stall speed. Of course, helicopters can be used to do this, albeit at a higher cost
So basically, the autogyro can do most of the things the helicopters/small GA aircraft can do at a lower cost. This low operational cost may be its main advantage.

I'm not aware of anything the autogyro can do which the others cannot do though.

The only thing I can think of that might be better about a gyrocopter is improved trailer-ability. Even that one is arguable though.

The ability to make landing with no ground roll is also a useful safety feature, especially useful for the low altitude operations where you have limited choice of landing spots if the engine quits.

I also want to mention these advantages (in addition to the lower operational cost):

Easy to fly and enjoy the flying experience.
Because it is in constant autorotation it can't stall.
It's simpler than a helicopter or plane
Because it's simpler and can't stall: With a proper design It can be safer than a helicopter or plane.

Mix that and you find the perfect recipe for sport/pleasure flying for people that have been dreaming of flying. Also this is the foundation of the bad safety reputation (with is not the fault of autogyros) as amateurs are building and flying this aircraft without the proper preparation.
Also:

With a prerotator and rotor pitch control it can do jump takeoffs, taking off vertically.
It can not hover but it can land within a very short distance (practically none)

In conclusion, private usage is the perfect application for Autogyros.

@Jan Hudec Rotor RPM will not suddenly drop, RPM will decay in a controllable maner so you can safely land.

Also, how is it simpler than fixed wing aircraft? It has engine similar to fixed wing aircraft, but instead of fixed wings and fixed stabilizers with rather simple control surfaces on them it has another rotating part controlled via rather complicated swashplate. And one additional control axis, the collective.

@Jan Huden You don't have Aileron, Flaps and Trim controllers. You only need a ruder and a teeter-head system wich is not that complicated. Collective pitch controls are not usually fitted to autogyros, they don't need it to fly.

I am not sure I would call any rotating component simpler than ailerons and elevators that are controlled by simple pulley.

Ok, without a collective it probably really can't stall. Doesn't the lack of it complicate the landing somewhat though?

Anyone can home-make a teeter-head system: (http://i35.tinypic.com/zn4qvo.jpg), I prefer to have one point of failure than multiple-ones that can make me die. It can land in a very very short distance https://youtu.be/UV3QPiX2uco so no, landing is not complicated: In fact in an autogyro it's easier than the take off.

The gyrocopter is cheap, versatile, and as easy to operate as an off-road motorcycle. What can it be used for? For everyone's daily ride to work.
The owner of Quobba Station in Western Australia owns a gyrocopter and uses it to patrol his land, and to muster the sheep and goats when time comes to sell them. He operates a barren "sheep station" the size of a small country, and has built the gyro himself.
A Robinson R22 would be prohibitively expensive. A small plane would not be able to land anywhere on his property. The off-road motorcycle ride is slower and less comfortable.


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What is the relation between throttle position and engine thrust for a turbofan?
I wonder how is designed the command chain from the throttle lever or the auto-throttle to the fuel injection in recent jetliners. I have 4 questions:
What is set by the throttle?

When the pilot actuates the throttle, what are they setting exactly? (I can imagine the lever position corresponds to a fuel rate, to a RPM, to a thrust value, etc).
Is this value a linear function of the throttle position (e.g. 73% of the full lever rotation is 73% of the maximum N1).

Is the throttle output standardized or customized for an engine model?

What type of output has the throttle lever: mechanical, electric, digital data?
Which subsystems in the engine receive the throttle output(s)?
Is this output customized/calibrated for an engine model, or standardized.

How are the throttle outputs processed in the engine?

Does the electronic engine control apply further adjustment to the throttle input?

How does the auto-throttle (auto-thrust) interact with the manual throttle?

Does the auto-throttle send its command directly to the engine or to the mechanical throttle?
Is the manual throttle position adjusted, according to the A/T output?

I don't understand why this is being voted down?

I think these are good questions but perhaps could be improved by being more specific. These things have changed over time and are not always the same between designs.

Check out this Nasa powerpoint it provides a nice overview of some of the basics and how some of the systems are set up.

Does the auto-throttle send its command directly to the EEC or to the
  mechanical throttle?

This may vary from design to design but the Boeing 777 uses the ARNIC standard bus to send control signals to servos that physically move the throttle levers. You can read up on the ARNIC 429 data bus here.

Are the throttle levers customized/calibrated for an engine model, or
  are their output standardized (assuming the output is an electric
  signal). Does the electronic engine control apply further adjustment
  to the throttle input?

It seems that Rockwell Collins makes a unit that sends signals to the FADEC system instead of mechanically actuating anything.
With that in mind Im sure there are older designs out there that are closer to or a complete direct linkage from lever to engine.

Is this correction transmitted back to the throttle lever?

I dont think so, it seems that when say full throttle is applied the throttle control will remain at full and the FADEC unit will take care of spooling it up as need be. Likewise if an engine suddenly cuts out or fuel flow becomes restricted as far as I know the throttles will remain in place. Auto throttle units will however physically actuate the throttles as need be. As per the comments here it seems this is only the case in Boeing aircraft. Airbus throttles must be set to maximum setting auto-throttle is allowed to use.

Auto-thrust units physically actuate the thrust levers in Boeing aircraft. In Airbus aircraft they don't (the thrust levers set the maximum A/T is allowed to use).

Thanks for the many elements provided, and Nasa's overview is very helpful.

What is set by the throttle?

For the commercial high-bypass turbofans that I am familiar with, the throttle sets a corrected N1 rpm (https://en.wikipedia.org/wiki/Corrected_speed if you aren't familar with corrected speeds).  Thrust is not linear with Fan RPM, but its not too far off (for high pass turbofan anyway). This thread may help What is the relationship between thrust and engine RPM in a turbojet?

Good additions, thanks.


text:
What is the relation between number of crew members (pilots and attendants) and fleet size in airlines?
Taking into account relevant elements like the fleet size or the type of flights (short or long haul, passengers or cargo, ...), do airlines use typical ratios between the number of aircraft and the number of pilots and attendants?

Clarification for the type of elements I'm looking for:

Northwings has 120 passenger aircraft and 720 pilots, this is comparable to other US airlines with modern aircraft (6 per passenger aircraft).
There are between 4 and 10 Captains per passenger aircraft. This is dependent on local labor laws and unions.
4 to 6 F/O for cargo aircraft (and no attendants because no passengers).
A380 are atypical at Thing Air with 700 flight attendants for 10 aircraft of that type.
Count 25% less for low cost airlines in the Middle East due to less restrictive labor laws and extended working time.

(Figures are for illustration, I've no idea of what the actual values are.)

I suspect that it varies considerably from airline to airline. I know of a low cost carrier who, from memory, had about 300 operational staff (not including head-office people like HR and finance) for a fleet of 8 narrow-bodied aircraft.

In addition to the parameters you already quoted, there's also the mix of aircraft types (more people needed for an A380 than for an 757), the mix of long-haul/very long haul flights (which require multiple crews on board) and country-specific parameters such as wages and regulations (on maximum number of hours per flight/per period, rest periods, etc.).

This seems like a remarkably broad question, but after chewing on it a bit and reading the 2 answers (so far), it's grown to be much more reasonable! +1

My answer may not directly answer your question but from MIT's The Airline Data Project, I extract data for year 2014 and put in table as follow:
Employees Per Aircraft                                  Southwest    United    American Airlines   Delta Airlines    Jetblue Airways
----------------------------------------------------------------------------------------------------------------------------------------
Pilots & Co-Pilots (Crews)                              6.0          8.3       6.8                 7.4               6.5
Flight Attendants (Crews)                               5.9          8.8       7.3                 6.8               5.0
Maintenance (Inhouse)                                   3.5          10.5      17.4                11.4              3.8
Passenger, Cargo and Aircraft Handling (Inhouse)        29.2         63.9      30.7                36.7              27.1
Other (Inhouse)                                         6.9          6.9       10.8                18.1              10.0
----------------------------------------------------------------------------------------------------------------------------------------
Total                                                   70.9         132.2     98.1                107.5             68.2

note that total number of crew does not add up with each type of crew in the table
Hope it gives you some idea about the ratio of pilot and flight attendant

6-8 pilots per aircraft is too low. That would be a reasonable # of captains/ aircraft, plus again that many FO's per aircraft. Since different planes require varying FA crew sizes, I don't know what the conversion should be there.  Those "total" numbers look reasonable as in employees/aircraft, but the details have some sort of other, missing, conversion factors.

@RalphJ, I think the unit is "crews". The flight deck crew is typically two persons, the number of flight attendants in a crew varies between aircraft types.

@DeltaLima  I'd buy that answer. Need to know the size of the FA crew in each case to get a meaningful answer to this question, obviously. Elsewhere in the MIT data, perhaps?


text:
Statistics: is being a crew on airliners overall roughly as dangerous as driving daily?
The question says it all. It's a commonplace that "airliner travel" for passengers is far and away the safest travel, on almost any metric (per mile, hour, per human-year, etc).
However, I've seen it said that for airline crew (who obviously fly ~once or even many times per day) the danger becomes as high as say "driving in Europe".
(I don't know how you quantify that - perhaps "deaths per year".  Also, I would assume the implication is versus "consumer car drivers", not versus eg truck drivers.)
This question is strictly about airliner† travel, i.e. large 20+ seat aircraft flying scheduled routes for name-brand national and international airlines: I don't want to see the statistics polluted with GA, private planes, bush flying, Fedex, rock band aircraft crashes from the 70s, etc.
Any ideas on this?  is it

Just a myth?

A distortion of statistics? (Something like "sure, crew obviously die more than passengers, but taxi drivers proportionately die spectacularly more than crew" - sort of thing?)

†["airliner" is defined end of story in the tags on this site, but I just thought I'd spell it out to avoid a rash of confusion]

PS I assumed this would have been fully addressed on here already; but couldn't find it.

Origin of the comparison between driving and frequent flying of flight attendants

The elevated radiation at high altitude is probably more of a health risk than a deadly crash.

@PeterKämpf Indeed!  I thought that might be the case (I didn't bother explicitly stating "crashes" since it seems implicit in the car case that radiation is not involved (except perhaps at https://en.wikipedia.org/wiki/Umling_La ))

As @PeterKämpf says, the figures about crashes are likely to be a sideshow. The health impacts of increased radiation and other impacts of constant travel are likely to be much more significant.

Here's how I look at it. You're at more risk to be in a car crash than a plane crash, but a plane crash presents a higher risk of death or grave injury. So, a car crash is less likely to kill you but significantly more likely to occur. Does it even the balance? I don't know.

Not an easy question. These statistics are scattered and hard to process.
One of the best summaries I've seen is here. It's not official, but the airline stats check against the original source. For GA, numbers have to be converted from 0.77 per 100k flight-hours to 35 per 1B miles at an average of 215 mph. For convenience, I'll use fatalities per trillion pax-miles or TPM. Fatality rates for crew tend to be a bit lower than for passengers, but not by far.
It comes down to:

70 per TPM for airliners; numbers are pre-MAX and exclude 9/11
110 per TPM for buses
150 per TPM for trains
7,300 per TPM for cars
~35,000 per TPM for GA
212,000 per TPM for motorcycles

That paints a picture of a 1:100 ratio of airline to car fatalities.
That said, the average US car driver covers 37 miles per day. The average FA does 90 hours per month, which at 500 mph comes to 1,500 miles per day. So the claim isn't 100% true - a daily car commute in the US is still 2.5x more dangerous than a flight crew job. But the US averages more traffic fatalities than Europe with its shorter commutes.
This doesn't compare flying all day with driving all day, only flying as a job with a simple daily commute. That said, professional drivers do build up skill resulting in lower accident rates. Buses have great stats. Taxi stats have been polluted by modern taxis being primarily ride-hail services with many part-time drivers, but even they average a lower accident rate than car commuters.
The real myth is that road driving is an exception from the rule that proficiency takes time and effort to achieve. The statistics say it's not. Professional drivers are better and safer than amateur drivers by almost as wide a margin (1:70) as that between amateur GA pilots and professional ATPs (1:500). People who drive for a living are in fact better at it than the rest of us.
A confounding factor is that airliners, even ones from less-reputable builders or in less-reputable airlines, are designed, built and maintained to incomparably higher standards than GA airplanes. This definitely plays into that 1:500 ratio. Between cars and buses, though, the difference in build and service quality is less drastic.
GA's safety record is comparable to other extreme sports and on the low end of the extreme, similar to scuba diving. It's safer than skydiving, mountaineering, or motorcycling. But not in the range of common carrier safety. Part of the reason is lack of hardware redundancy, a larger part is a low skill threshold. It's a sport that can also be a means of transportation, not the other way around.

What happens more often: a fatal crash, or a flight being cancelled/delayed because the pilot died in a car crash during the commute to the airport?

@DeltaLima Is the plane a [s]Calhoun[/s] Boeing? Is the car a Pinto? Is the airline based in a LDC?

Also a factor when comparing TPM - the typical bus might seat 50-ish passengers. There are smaller planes in scheduled service, but when you're looking at typical Boeing and Airbus airliners in scheduled service they are more like 100 - 400 passenger capacity.

@Fattie Among extreme sports, GA's mortality is far from the highest, and similar to scuba diving. But the right way to treat it, in my view, is as a sport, with its associated risks.

Why exclude 911?

@jpe61 The 9/11 crashes (and any other bombing, hijacking, missiles, etc.) are arguably a "danger" of airliner travel. But my sense of the question - and I suspect others as well - is that this question is about safety issues due to weather, mechanical and human (e.g., pilot mistakes) factors independent of hostile actions from another party (whether terrorist or nation-state).

As far as "if GA is so dangerous, why not get rid of GA?", one of the key uses of GA is as a way for pilots to develop their skills. Arguably if GA was not an option then the only pilots would be ex-military, and we wouldn't have nearly enough pilots - from what I have read there is a chronic shortage even with GA.

@manassehkatz-Moving2Codidact acts of terrorism are an integral part of the risks involved with commercial aviation. They should not be excluded when considering how dangerous flying is.

@Fattie why would 1:500 make you want to "end GA"? Are you implying it is your (or a regulator's) job to tell a GA pilot what risk (personally and knowingly) they are allowed to take? Maybe while we are at it, we should ban driving, because "too many" drivers end up killing themselves? There is a list of things we could do to make GA safer (getting pilots an IR rating, turbine engine(s!), anti-ice, real-time weather radar) but they all take money, and many GA pilots decide piston single VFR has an acceptable safety/cost balance. (Go be an aero engineer if you want to change the balance.)

hi @Azendale I was really just making a hyperbolic or humorous point "FIVE HUNDRED X?!"  However (even within that humorous context: ) note that as a card-carrying libertarian I couldn't care less if someone kills themselves rock climbing, flying etc. ... the point is if they kill people on the ground.  Actually I was about to comment "Thank goodness when GA planes crash, it seems they very rarely hit suburbs, towns, houses ..."  I'm not an aero engineer but I did work at the BMW-RR aero engine plant !!

Scuba diving should be banned though - it's too dangerous

I think this view fails to place enough emphasis on the elimination of variables (and variable behavior) that makes both airliners and buses "safer." Airliners are glorified buses, following well-established procedures and routes over and over, with no capricious variations that invite mistakes or mishaps. Individual car drivers take highly variable trips, changing lanes at will, amidst thousands of others doing the same thing. Likewise GA pilots are often maneuvering at will on arbitrary courses.

@Oscar This would make for a decent separate question. That said, I would disagree: weather (for aircraft) and traffic (for cars) are much more important variations than the exact route being taken.

@Therac It's not about the exact route. It's about the SAME route and same procedures, with no off-the-cuff variation allowed. Check out top causes of GA accidents: https://generalaviationnews.com/2019/06/24/top-10-causes-of-fatal-ga-accidents/  I'd say at least half of those are essentially eliminated by strict adherence to (and repetition of) procedures.

@Oscar I think this is worth a separate question.

From What percentage of airplanes are involved in a crash in their lifetime?, mean values derived from IATA 2014 statistics (IATA is the air carriers association):

IATA aircraft in the World: 23.000.
Flights per day per aircraft = 4.3.
A fatal accident after 3 millions flights.

You may read the linked question to see how we get to the following outcome:

A fictive airline with 320 aircraft, which is the size of an operator like Air France, faces one fatal accident each 6 years.

Now deriving new numbers from this result.

New assumptions:

A career is 40 years.
A crew works 5 days a week
5 crews are required to cover 24 hours, thus each crew flies 4.8 hours a day. Thanks to @DeltaLima for providing this ratio.

From this:
$
\sf \small {
\begin {array}{|l|l|l|}
\hline
\sf \small \text {Years between fatal accidents for the airline}
&
& \sf 6
\\ \sf \small \text {Daily probability for the airline}
& \sf 1 / (6 \times 365)
& \sf 4.57e-04
\\ \sf \small \text {Daily probability for a single aircraft}
& \sf (4.57e-04) / 320
& \sf 1.43e-06
\\ \sf \small \text {Daily probability for a crew member}
& \sf (1.43e-06) / (7/5) / (24/4.8)
& \sf 2.04e-07
\\ \sf \small \text {Yearly probability for a crew member}
& \sf (2.04e-07) \times 365
& \sf 7.44e-05
\\ \sf \small \text {Career probability for a crew member}
& \sf (7.44e-05) \times 40
& \sf 2.98e-03
\\ \sf \small \text {Years worked before death}
& \sf 1 / (2.98e-03)
& \sf 1.34e+04
\end {array}
}
$
During their career, a crew member has a probability of dying in a crash of 0,298%. It means the probability is 100% after 13,400 years.
While I take a fictive airline of 320 aircraft, these final figures are mean values for any airline size. But the distribution of accidents is not flat, it depends also on local factors, e.g. how maintenance and training are delivered.

See also: What are the statistical probabilities of commercial aircraft accidents?

Fascinating. Completely randomly googled factoid "The lifetime odds of dying in a crash for a person born in 2021 are 1 in 93" supposedly. (As always, statistics from any source are often totally nonsensical since very few people understand statistics but everyone thinks they do, but I just randomly googled that up, it's an interesting approach to stating the likelyhood.)

Here's a probably reasonably reliable source for "deaths per 100,000 humans" (blue line) https://injuryfacts.nsc.org/motor-vehicle/historical-fatality-trends/deaths-and-rates/  Note though that the second chart, scroll down, deaders per mile, blue line, is drastically different.

Hi @mins - indeed I ranted about that extensively in the question.

I'm in my late 60s so this 5700 years looks like a good bet for me risk wise.

@Fattie: "The lifetime odds of dying in a crash for a person born in 2021 are 1 in 93". I don't understand this figure. Overall people fly less than once a year, especially people born in 2021. Does it mean flying 50 times a year correspond a likelihood of more than 50/93 of dying?

@mins - back up!  that "The lifetime odds of dying in a crash for a person born in 2021 are 1 in 93" was in answer to your question about CARS.  Crap, I forgot the link, https://www.injurylawyers.com/blog/the-odds-of-dying-in-a-car-crash/  (purely ftr, since it's not 1950, you can now just speak any setence like that in to the air and your device will give you the link!)  Sorry for the confusion !!

Re "So no crew from this operator could die before flying 6 years"  --surely you didn't really mean to say that?

@quietflyer: If for this airline fatal accidents have a mean period of 6 years, for the 6 first years, nobody died, the next day the whole crew and the passengers died. And don't call me Shirley :-)

@mins I am trying to understand your logic in your calculation. You divide by a factor 8 to arrive at 5700 years. Are you assuming average 8 hour working days? I think that is extremely high. In staff planning we need approximately 5 FTE for 1 position in 24/7 operations. You need 3 people to work around the clock in 8 hour shift, but you also have to account for weekends, sick leave,  holidays, training etc.

@DeltaLima: Correct, 8hrs a day. I wasn't aware of the flight time, I've indicated that as well as other elements. It was just to estimate a maximum likelihood (or minimum period). So with 4.8 hrs, it's more than 9200 hrs between accidents, and 0.4% in a career. By refining other elements I'd not be surprised to get less than 0.1%. Thanks for the info.

For pilots, FAA has a regulatory limit of 1,000 flight hours per year (FAR 121.481(f)).

".... It means the probability is 100% after 13,400 years." This statistical statement is obviously wrong and hints that you should rethink how this conclusion was derived. To make it more clear: Just because the chance of rolling a 6 on a die is 1/6, the chance to roll at least one 6 after 6 rolls is not 100%. After any finite number of rolls it will in fact never be 100%, instead it is 1-(5/6)^n, where n is the number of rolls.

@GWD: It's true for the dices, because the probability is not conditional, however if you have a prior knowledge there is a 6 each 6 rolls, then after 6 rolls the probability is 1. In my case I stated there is an accident each 6 years for a 320-aircraft fleet.

@mins There’s no law requiring a crash every 6 years. If you say there’s an average 1 crash in 6 years, then it’s exactly the same as the dice: you roll a 6 once every 6 rolls on average. But not with 100% probability, not at all.


text:
How do pilots and controllers deal with TFR's that lie directly on an approach?
FDC Notam 4/3621 creates a generic "template" TFR which sits on top of sporting events.  This one in Phonenix appears to sit right on one of the business ends of Phoenix Sky Harbor (PHX)  The Notam states that event operations are permitted with the appropriate waiver.  How do regularly scheduled airlines deal with such TFR's?  Are waivers obtained for scheduled inbound flights?  There does not seem to be verbage in the notam granting any exceptions for air carriers.

Ahh, there is verbiage in the NOTAM for being in contact with ATC.  I'll try to read more slowly next time, and call up the tower when I want to sight-see over the football game in in my town, almost like old times.

The NOTAM itself answers your question:

THE RESTRICTIONS DESCRIBED ABOVE DO NOT APPLY TO THOSE AIRCRAFT
  AUTHORIZED BY AND IN CONTACT WITH ATC FOR OPERATIONAL OR SAFETY OF
  FLIGHT PURPOSES, DEPARTMENT OF DEFENSE, LAW ENFORCEMENT, AND AIR
  AMBULANCE FLIGHT OPERATIONS.

As long as the pilot is in contact with Air Traffic Control and has been cleared through the TFR, they can safely fly though the airspace.

There is exemption language in FDC 4/3621 (note the underlined section in the screen grab below):

THE RESTRICTIONS DESCRIBED ABOVE DO NOT APPLY TO THOSE AIRCRAFT AUTHORIZED BY AND IN CONTACT WITH ATC FOR OPERATIONAL OR SAFETY OF FLIGHT PURPOSES

Conducting an approach (instrument or visual) to an airport is an "operational purpose", and as KPHX is in Class B airspace with an operating control tower any aircraft landing there will, by necessity, be in contact with ATC (or in violation of a bunch of other regulations).
If ATC tells you to fly through the stadium TFR for your approach and landing you then meet the requirements of that exemption ("authorized by and in contact with ATC", for the operational purpose of landing at Sky Harbor)

Note that this was not always the case.  The earliest versions of the "sporting events TFR" did not include exemptions for aircraft in contact with ATC (that appeared in 2009, presumably because the FAA realized that requiring every airline flying into Phoenix to get a waiver for baseball season was an administrative nightmare for all concerned).

The City of Seattle has the major Baseball and Football stadiums directly in the approach path for Boeing Field.
The TFR covers both KBFI's Class D and KSEA's Class B airspace.
The text of the NOTAM specifically says that when talking to controllers, you can go through it.  That is not only for the purpose of approach the airports.  You can also request a transition, and so long as you're talking to ATC and following their directions, you can go right through the TFR.
I believe I've even observed sight-seeing flights circling the stadium, rather than transitioning the space.  Presumably they were talking to ATC and got permission.   The point is, depending on the text of a TFR, the restriction may not be inviolable.
Some TFRs, such as a Presidential TFR during arrival and departure procedures will be rock-solid.

The Presidential TFRs do still have an exception for regularly scheduled commercial passenger and all-cargo carriers operating under one of the following TSA-Approved standard security programs/procedures: aircraft operator standard security program (AOSSP), full all-cargo aircraft operator standard security program (FACAOSSP), model security program (MSP), twelve five standard security program (TFSSP) all cargo, or all-cargo international security procedure (ACISP) and are arriving into and/or departing from 14 cfr part 139 airports., though.

^ Quoted from the current TFR at Martha's Vineyard, but appeared in all Presidential TFRs I've read.

There are parts of a Presidential TFR that are rock-solid, such as when AF1 is arriving/departing.  During those times, all ground movement is stopped, and the airspace is completely cleared.   You are correct that once AF1 is sitting on the ground, certain operations are allowed.

Oh, yeah, it's basically a no-fly zone when AF is actually arriving or departing. It also creates a mess on the ground until the motorcade actually leaves the airport. On the other hand, it can yield a nice view of AF1 pulling up on the ramp.


text:
What are cowl flaps?
What are cowl flaps and what are they used for? It seems I have only heard of them in connection with old planes, are they peculiar to radial engines?

They are flaps or doors on the engine cowling that open up in order to provide increased cooling airflow for the engine. They look like this when open. They are generally controlled by a handle in the cockpit (some aircraft have electric cowl flaps with an open/close switch).

The disadvantage is that the open flaps also increase drag. So on airplanes equipped with cowl flaps, they are generally opened for low-speed, high-power situations such as takeoff. This is the harshest operational regime for the engine since it is at its highest power and relatively low cooling airflow due to low airspeed. They are then closed either reaching cruise or reaching a point where the airplane can transition to a lower-power, higher-speed climb profile.
They're not specific to radial engines, but do tend to be more common in older aircraft (there are exceptions though). Newer airplane and engine designs are more likely to have sufficient inherent cooling airflow to not require cowl flaps.
The photo above comes from AVweb, which also has a nice article explaining more about the function and usage of cowl flaps.

Cowl flaps are used for engine cooling and you'll find them on many high performance piston aircraft.  They are particularly useful during climbout when you are at a high power setting and slow(ish) airspeed.  Opening the cowl flaps allows more airflow across the engine but produces some drag.  They are typically mounted on the bottom of the engine cowling.